The present invention relates to the manufacture of sheet materials such as paper, and more particularly to the control of the sheet basis weight. Still more particularly, the present invention relates to a system and process for adjusting the slice lip opening of a head box in a paper making machine.
Certain sheet materials are manufactured from starting material which is in a fluid or paste-like state. The manufacturing process for such materials typically involves controlling the flow of the starting material onto a conveyer belt or the like. FIG. 1 is a cross-sectional view of part of an exemplary device used in connection with paper manufacturing. A slurry of paper pulp 10, containing wood pulp fibers in suspension as well as mineral fillers, flows from a head box (only a portion of the headbox is shown as frame 12) onto a moving porous conveyor belt 14 on which the slurry 10 forms a continuous sheet 16 moving in the direction of arrow 26. A thickness-regulating device is used to regulate the flow of the slurry 10 onto the belt 14. The thickness regulating device includes a deformable thickness regulating member called a "slice lip" 18 and a rigid member 20 located at the bottom of the head box frame 12. This rigid member 20 is known as a "forming board". Both the slice lip 18 and forming board 20 span the width of the sheet 16 in the "cross-direction" (i.e. The direction across the width of the sheet perpendicular to the direction of sheet movement). The adjacent edges of the slice lip 18 and forming board 20 are disposed close to one another, thereby defining a space 22 through which the slurry 10 discharges from the head box 12 onto the conveyor belt 14. The slurry 10 dries as it is carried along the belt 14 and thereafter the sheet is typically passed through a system of rollers (not shown) for the removal of additional moisture, and further processing.
The basis weight of the sheet 16 (i.e., the mass of slurry per unit of sheet surface area) is dependent in part on the height of the slice lip opening 22. Typically, the forming board 20 is fixed while the lower edge of the slice lip 18 is movable relative to the forming board 20 to adjust the height of the opening 22. The movement of the slice lip 18 is typically controlled by a number of actuators 24 (only one shown) which are referred to in the paper making art as slice rods. One end of each slice rod 24 is connected to the slice lip 18 near the opening 22 and the other end of each slice rod 24 is connected to the head box frame 12. Typically, a plurality of such slice rods 24 are used. These slice rods 24 are spaced apart from each other at fixed intervals along the length of the slice lip 18 in the cross-direction. The slice lip 18 is deformable under the actions of the slice rods 24 which can be individually adjusted to control the relative displacement of different segments of the slice lip 18, as explained more fully, for example, in commonly assigned U.S. Pat. No. 4,680,089 to Aral et al. The Aral patent is incorporated herein by reference.
In the past, the movements of the slice rods 24 were controlled by manually operated screw jacks 25 which pushed the slice rods 24 against the slice lip 18 (see, for example, FIG. 1). Alternatively, electrical heating coils (not shown in FIG. 1) may be disposed inside the slice rods 24 for changing the temperature of the slice rods 24 and thereby changing the lengths of these rods by thermal expansion and contraction. The length of each slice rod 24 may thus be controlled by controlling the amount of electric current supplied to the corresponding heating coils. The change in rod length changes the height of the opening 22 near the rod 24. Therefore, the heating coils may be individually controlled to displace different segments of the slice lip 18 by different amounts.
The displacement of the slice lip 18, and thus the height of the opening 22, could be determined using a displacement measuring sensor (not shown) coupled to the lower edge of the slice lip 18. For example, the slice lip displacement could be determined by using a linearly variable differential transformer ("LVDT", not shown) coupled between the lower edge of the slice lip 18 and the head box frame 12. LVDTs are well known in the art of measuring displacements. The method of determining the slice lip position using LVDTs is, however, subject to certain limitations and inefficiencies. For example, LVDTs are rather delicate position sensing devices having sliding parts. Because the LVDTs would have to be connected in close proximity to the slurry 10, slurry particles and moisture from the slurry may cover the sliding surfaces, thus causing sticking, jamming and corrosion of the sliding parts.
Another limitation associated with the use of LVDTs to measure slice lip displacements is the high cost involved. It has been found that an accuracy of 0.001 inch is required in many applications, for example, paper making. A relatively expensive, high resolution LVDT is therefore required. The use of LVDTs would be especially costly to a paper manufacturer since the expected LVDT service life is short in the harsh operating environment of a paper mill. The cost further increases when the slice lip displacement is to be determined, as would be required in a paper mill, at a plurality of locations along the the slice lip, thus requiring a plurality of such expensive LVDTs.